Problem: the Norks actually are starving, have a poor but fairly large standing army, and are sitting on China's border, and are not estranged from South Korea, on a personal level. China would prefer not to have a humanitarian catastrophe on it's borders, starving people looking for food, causing mayhem. Gunning the starvies down antagonizes the South, needlessly, and of course, having an unstable state actor as a loose ally is not the worst thing in the world. Iran has Hezbollah. China has the Norks. America... spends a lot of time vetoing UN resolutions against allies in the Levant, do they not?